How do people decide which claims should be considered mere beliefs and which count as
knowledge? Although little is known about how people attribute knowledge to others,
philosophical debate about the nature of knowledge may provide a starting point. Traditionally, a belief that is both true and justiﬁed was thought to constitute knowledge. However,
philosophers now agree that this account is inadequate, due largely to a class of counterexamples (termed ‘‘Gettier cases’’) in which a person’s justiﬁed belief is true, but only due to
luck. We report four experiments examining the effect of truth, justiﬁcation, and ‘‘Gettiering’’ on people’s knowledge attributions. These experiments show that: (1) people attribute
knowledge to others only when their beliefs are both true and justiﬁed; (2) in contrast to
contemporary philosophers, people also attribute knowledge to others in Gettier situations;
and (3) knowledge is not attributed in one class of Gettier cases, but only because the agent’s
belief is based on ‘‘apparent’’ evidence. These ﬁndings suggest that the lay concept of knowledge is roughly consistent with the traditional account of knowledge as justiﬁed true belief,
and also point to a major difference between the epistemic intuitions of laypeople and those
of philosophers.